This invention relates to a process for producing an acrylamide series polymer having good solubility and high molecular weight.
In recent years, a water-soluble acrylamide series polymer has been used in various fields such as coagulants, petroleum recovering agents, binders for paper formation, paper force-reinforcing agents, etc. in large amounts. Of these uses, coagulants, petroleum recovering agents and binders for paper formation are often required to be high molecular weight and those having an average molecular weight of 10,000,000 are not unusual. As a process for obtaining such a high molecular weight acrylamide series polymer, various proposals have been made but the method in which polymerization is carried out by using a free radical initiator in an aqueous medium is industrially most popular. In this case, a water-containing polymer obtained generally contains several ten % or more of water but the molecular weight or the polymer is too high so that it is a rubber like material of a viscous liquid which does not substantially flow. Accordingly, handling of these materials is difficult as such and transportation is also uneconomical, and thus, they are generally used in the form of dry powder by removing water from the above water-containing polymer by hot-air drying, etc.
However, solubility of the dried polymer obtained by heat drying method in water becomes generally bad as the concentration of the monomer at polymerization, the molecular weight of the polymer and drying temperature are increased. This lowering in solubility can be improved to a certain extent by elongating the time of dissolution at use. However, in extreme cases, particles which merely swell and do not dissolve are present in a large amount even when they are stirred for a long period of time in water. Thus, when it is applied to waste water processing, etc. as a coagulant, it shows low coagulating property. Also, there are problems that, when it is used as a petroleum recovering agent, penetrating property is lowered and when it is used as a viscosity increasing agent for papermaking fish eye is generated on a paper prepared therewith.
On the other hand, as a stabilizing agent for an acrylamide polymer, it has been known that 2-mercaptobenzothiazole is effective as disclosed in Japanese Patent Publication No. 27659/1974 and there is described that the compound is added to the polymer after polymerization has completed. Also, in this publication, while there is a description suggesting to co-present the above stabilizer at polymerization, it is not specifically disclosed. Mercaptanes have been known rather as a molecular weight controller as described in "High Molecule Chemistry, the first volume" (written by P. J. Flory, translated by Oka and Kanamaru, published by Maruzene Co., October 1955), page 137, so that if mercaptanes such as 2-mercaptobenzothiazole, etc. are added during polymerization, it has been considered that the molecular weight is lowered by its chain transfer effect whereby high molecular weight product can hardly be obtained.
Also, in Japanese Patent Publication No.8688/1985, there is described that 2-mercaptobenzoimidazole which has similar structure as that of 2-mercaptobenzothiazole is added during polymerization, but an amount thereof added in Example is 0.05% by weight or less based on the monomer. According to the present inventors investigation, it can be confirmed that when it is added with an amount of 0.5% by weight, polymerization does not start at all.
As described above, a process for producing a dried acrylamide type polymer having a large molecular weight and good solubility in water with efficiency is demanded but a technique which satisfies these conditions has not yet been developed at present.